What does this stress look like? PH lockout? Overwater? Natural Fade?

Normal that late in bloom, if near done. You can keep a canopy mostly green up through the end. Feed a bit more and it stays green longer. simply provide what it needs, properly. if you have been feeding constantly, back off the pedal a bit, especially in soil. It retains it longer than other mediums. if you were causing any toxicities, you'd know from what you are doing to the medium. I don't think it's waterlog droop though. If it is, it'll go away soon and let it dry top 1/2 to 1" or so (soil, anyway). weight suggestion is even better. Some people like to flush or purposely cause a big fade at end, but you have to time it - which is better with a clone or known quantity of course. Otherwise, you just hurt your yield for a bunch of hocus pocus and 'bro science.' Various tissues are diversified in their function. unless you find cells mixed with calyxes and pistils, it's not storing excess anything there. As far as most of your questions, you'd have to test such things and if the meter says it's too high or too low, yo have an answer. I can't tell very well in pictures, but your buds look fairly close to done, if not mistaken. The decimated canopy may not be a problem as long as you can harvest soon. You still have lots of green to give, so it's still fine. If you think it has a long time to go, stop flushing if you are already, and if not delay it a bit longer than normal. autumn-like leaves can be caused by slightly cool temps at night - maybe 65F and increased rate the furtherbelow that you go. check your runoff and see if it is lower or higher -- note that this is not your soil's pH but can be an indication it is too lower or high based on how drastically different the runoff is from what you put in. Same with a soil slurry. It may be a bit more precise, but it isn't accurate; or swap those around; you get the idea.